System and method for embedding OFDM in CDMA systems

ABSTRACT

Systems and methods of combining OFDM and CDMA signals are provided. An OFDM packet data channel is overlaid over CDMA transmissions. The channel is scheduled slotwise between multiple users. In some embodiments, there is a CDMA packet data channel which is scheduled together with the OFDM packet data channel.

RELATED APPLICATIONS

This application claims the benefits of prior U.S. provisionalapplication No. 60/493,800 filed Aug. 11, 2003 and No. 60/494,087 filedAug. 12, 2003, hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to single and multi-carrier CDMA systems andmethods, and to methods of providing increased capacity to such systemsand methods.

BACKGROUND OF THE INVENTION

Two existing CDMA standards include UMTS, also referred to as WCDMA and3 GPP, and CDMA 2000 also referred to as 3GPP-2.

Referring firstly to FIG. 1, shown is an example of a conventional WCDMA(wideband code division multiple access) downlink structure. Thisconsists of a number of channels all of which are separated using Walshcodes. These are shown to include a primary SCH (synchronizationchannel) 10, secondary SCH 12, pilot channel CPICH (common pilotchannel) 14 and a dedicated channel 16, for example a voice channel. Allof these are typically covered with a high spreading factor, for example64. There is a 10 ms frame containing 15 slots each 10/15 ms induration. Also shown is the WDCMA HSDPA (high speed data packet access)channel 18. This channel is used in a time division multiple accessfashion, with two millisecond slots being assigned on a scheduled basisto different users. One such 2 ms slot 26 is shown to contain three10/15 ms slots 20, 22, 24. This provides a fairly high bandwidthchannel. This is still transmitted using CDMA, but typically thespreading factor is lower than that used for the other channels, and itmight for example be 16 or 32. All of the channels are covered with acommon PN code. Advantageously, this structure is backwards compatiblewith existing UMTS terminals which do note use the WCDMA HSDPA channel.The WCDMA HSDPA is included in the so called EV/DV evolution.

Referring now to FIG. 14, shown as the structure of the conventionalCDMA2000 release A/B/C channel structure. This channel structureincludes release A\B channels 200 and release-C new channels 202. Therelease A\B channels 200 include a CDM pilot 204, a synch channel 206,broadcast channel 208, paging and quick paging 210, CACH 214, F-FCH 214,F-SCH 216, F-SPDCCH 218. The release-C new channels 204 include theF-SPDCCH 218 and the F-PDCH 220. The frame structure consists of a 20 msframe 222 divided into sixteen 1.25 ms slots 228. Each such slot is usedto transmit 1536 chips. Eight such slots produce a 12288 chiptransmission 230. Some of the channels are power-controlled channels asindicated at 224, and some of the channels are rate-controlled channelsas indicated at 226.

For both of the above examples of existing CDMA systems, the Walsh codespace available is all but depleted, and as such new methods ofproviding further bandwidth would be desirable.

SUMMARY OF THE INVENTION

According to one broad aspect, the invention provides a transmitteradapted to transmit a downlink signal comprising: at least one codeseparated CDMA (code division multiple access) channel(s) that arescrambled with a common scrambling code; an OFDM packet channel overlaidover the CMDA channels, the OFDM packet channel being divided into OFDMslots during which OFDM signals are transmitted with capacity allocatedon a per OFDM slot basis, the OFDM packet channel not being scrambled bythe common scrambling code.

In some embodiments, the at least one code separated CDMA channel(s)comprises at least one continuous CDMA channels and a CDMA packetchannel that is divided into CDMA slots during which CMDA signals aretransmitted with capacity allocated on a per CDMA slot basis.

In some embodiments, the at least one code separated CDMA channel(s)comprises a CDMA packet channel that is divided into CDMA slots duringwhich CMDA signals are transmitted with capacity allocated on a per CDMAslot basis.

In some embodiments, the transmitter is adapted to transmit packet dataon a sequence of slots, each slot being either an OFDM slot or a CDMAslot, but not both.

In some embodiments, the transmitter is adapted to transmit packet dataon a sequence of slots, each slot being an OFDM slot and/or a CDMA slot.

In some embodiments, the transmitter is further adapted to transmitperiodically at least one CDMA signalling channel that is not spread bythe scrambling code.

In some embodiments, during periods of overlap between the transmissionof the at least one signalling channel and the transmission of an OFDMslot, the OFDM slot transmits zeros or known data so that those periodscan be treated as a prefix for the OFDM slots upon reception.

In some embodiments, each slot comprises a prefix followed by an IFFTperiod, followed by a suffix.

In some embodiments, each OFDM slot is 10/15 ms in duration, and theprefix is 128 chips, the IFFT period is 2304 chips, and the suffix is128 chips.

In some embodiments, the IFFT period comprises a plurality of IFFTsseparated by zero insertions.

In some embodiments, the IFFT period comprises 18 128-chip IFFTs.

In some embodiments, the prefix is a designed training sequence.

In some embodiments, the at least one signalling channel is transmittedduring the prefix of the OFDM slots, and the OFDM slots transmit zerosduring the prefix so that the OFDM channel does not interfere with theat least one signalling channel.

In some embodiments, the transmitter is further adapted to transmit acontrol channel that identifies to receivers which OFDM slots and/orwhich CDMA slots are for a given receiver.

In some embodiments, the transmitter is adapted to transmit a controlchannel during the suffix that identifies to receivers which OFDM slotsand/or which CDMA slots are for a given receiver.

In some embodiments, the control channel contains a receiver specificmask and/or a receiver specific CRC code.

In some embodiments, each OFDM slot comprises in sequence: a firstprefix; a first IFFT period, a first suffix; a second prefix, a secondIFFT period, and a second suffix.

In some embodiments, each OFDM slot is 10/15 ms in duration, and eachprefix is 128 chips, each IFFT period comprises 16 64 chip IFFTs, andeach suffix is 128 chips.

In some embodiments, a system is provided comprising the transmitter incombination with a plurality of receivers at least one of which is anOFDM-capable receiver, wherein each OFDM-capable receiver demodulatesthe CDMA channels on an ongoing basis, and each OFDM-capable receiverdemodulates a given OFDM slot only if scheduled during the OFDM slot.

In some embodiments, a system is provided comprising the transmitter incombination with a plurality of receivers at least one of which is anOFDM-capable receiver, and each OFDM-capable receiver demodulates agiven OFDM slot only if scheduled during the OFDM slot, and eachreceiver demodulates a given CDMA slot only if scheduled during the CDMAslot.

In some embodiments, the transmitter further comprises a scheduleradapted to schedule receivers on the downlink by: for each of aplurality of OFDM-capable receivers, obtaining a respective channelquality indicator for CDMA slots and/or for OFDM slots; schedulingtransmission to OFDM-capable receivers on CDMA and/or OFDM slotsaccording to the channel quality indicators.

In some embodiments, for a given receiver, the channel quality indicatorfor CDMA slots and/or for OFDM slots comprises a better of the CDMAchannel quality indicator and the OFDM channel quality indicator asdetermined by the receiver.

In some embodiments, the scheduler is further adapted to: for CDMA-onlycapable receivers, obtain a channel quality indicator for the CDMA slotsonly, and schedule transmission to CDMA-only capable receivers on thebasis of the CDMA channel quality indicator.

In some embodiments, the transmitter is further adapted to: for at leastsome of the OFDM slots, partition OFDM sub-carriers transmitted duringat least some of the OFDM slots between a plurality of receivers on apartitioning period basis.

In some embodiments, a partitioning period comprises one OFDM slot.

In some embodiments, the transmitter is further adapted to transmit acontrol channel identifying how each OFDM slot is partitioned betweenreceivers such that each receiver can obtain its content.

In some embodiments, the control channel comprises: identifiers for eachreceiver scheduled during the slot and an indication of whichsub-carriers are for which receiver.

In some embodiments, the control channel comprises: a first sub-channeland a second sub-channel; the first sub-channel containing an identifierof a first receiver and an identifier of a last OFDM sub-carrier for thefirst receiver; the second sub-channel containing an identifier of asecond receiver, and an identifier of a last OFDM sub-carrier for thesecond receiver, the second sub-channel being only transmitted whenthere is a second receiver scheduled during a given slot.

In some embodiments, the transmitter is further adapted to performsub-carrier and power allocation.

In some embodiments, the transmitter is adapted to perform sub-carrierand power allocation dynamically as a function of mobility and/orfrequency selectivity.

In some embodiments, the transmitter is further adapted to performsub-carrier and power allocation dynamically as a function of mobilityand frequency selectivity by: defining conditions in which CDMA is usedinstead of OFDM as a function of channel quality and/or mobility and/orfrequency selectivity; when conditions indicate OFDM is to be used: a)using a narrow frequency band and high power for high mobility, highfrequency selectivity users; b) using a wide frequency band and lowpower for mid-mobility and mid-frequency selectivity users.

In some embodiments, the transmitter is further adapted to perform OFDMallocation subject to an overall power budget for an OFDM slot.

In some embodiments, the transmitter is adapted for use in a multiplecarrier CDMA system.

In some embodiments, the transmitter comprises: an OFDM modulatoradapted to generate a single set OFDM sub-carriers that spans themultiple carriers, and in which a single wideband IFFT is performed todo OFDM modulation.

In some embodiments, each carrier is allocated to only one of CDMA orOFDM on a per allocation period basis, and for any carrier that isallocated to CDMA during a given allocation period, zeros are insertedin a portion of the OFDM IFFT that overlaps the carrier during thatallocation period.

In some embodiments, the OFDM modulator is adapted to insert guardbandsub-carriers between groups of sub-carriers for each carrier.

In some embodiments, each OFDM slot comprises: a control portion whichis non-OFDM followed by a plurality of IFFTs each separated by arespective prefix.

In some embodiments, each IFFT is a 128 chip IFFT, and each prefix is 22zero chips.

In some embodiments, the transmitter is further adapted to: adjust atleast one power control parameter prior to transmitting OFDM slots so asto avoid/mitigate power control loop panic.

In some embodiments, the at least one power control parameter comprisesan outer loop power control threshold.

In some embodiments, the transmitter is adapted to preferentiallytransmit CDMA slots in flat fading conditions.

In some embodiments, the transmitter is adapted to preferentiallytransmit OFDM slots in dispersive conditions.

According to another broad aspect, the invention provides a receivercomprising: an A/D converter adapted to produce a sequence of samples ofreceived signal; CDMA demodulator for performing CDMA demodulation uponthe sequence of samples; OFDM demodulator for performing OFDMdemodulation upon the sequence of samples.

In some embodiments, the receiver is further adapted to receive acontrol channel identifying for a given slot whether or not the receiveris scheduled during the slot; wherein the receiver demodulates a givenOFDM slot only if scheduled during the slot.

In some embodiments, the receiver is further adapted to: determine aCDMA channel quality indicator for CDMA and an OFDM channel qualityindicator for OFDM; feed back a better of the CDMA channel qualityindicator and the OFDM channel quality indicator; if the CDMA channelquality is better, use CDMA mode to demodulate a next slot for thereceiver; if the OFDM channel quality is better, use OFDM mode todemodulate a next slot for the receiver.

In some embodiments, the receiver is adapted to: perform CDMAdemodulation by descrambling at least one common channel, de-spread andperform soft-de-mapping and FEC decoding during which the OFDM contentwill be substantially converted to AWGN; demodulate the OFDM slots bygenerating an interference term due to a CDMA component of the receivedsignal, subtract the interference term from the received signal, performOFDM demodulation on a remaining signal.

In some embodiments, the receiver is adapted to generate theinterference term by: de-scrambling, de-spreading, soft de-mapping andFEC decoding (or hard decision), re-encoding, re-spreading,re-scrambling, convolving with an estimated channel response.

In some embodiments, the receiver is adapted to use OFDM pilots togenerate channel estimates for both OFDM and CDMA.

In some embodiments, the receiver is adapted to perform channelestimation by: performing an FFT on the sequence of samples; performinga first transformation on a set of samples collectively output by theFFT; performing a per sub-carrier transformation on an output of thefirst transformation; performing a second transformation on outputs ofthe per sub-carrier transformations collectively to produce an OFDMchannel estimate; and performing an IFFT on the OFDM channel estimate toproduce a time domain channel estimate for CDMA finger searching.

In some embodiments, the receiver is adapted to use CDMA pilots forchannel estimation for both CDMA and OFDM.

In some embodiments, the receiver is adapted to perform channelestimation by: performing CDMA searching and correlation to produce atime-domain channel response; performing an FFT on the time domainchannel response; performing a first transformation on a set of samplescollectively output by the FFT; performing a per sub-carriertransformation on an output of the first transformation; performing asecond transformation on outputs of the per sub-carrier transformationscollectively to produce an OFDM channel estimate; and performing an IFFTon the OFDM channel estimate to produce a time domain channel estimatefor CDMA finger searching.

In some embodiments, the receiver is adapted to receive a signal onmultiple carriers, the receiver comprising an FFT function adapted toperform an FFT for sub-carriers spanning the multiple carriers.

According to another broad aspect, the invention provides a methodcomprising: generating at least one code separated CDMA (code divisionmultiple access) channel(s) that are scrambled with a common scramblingcode; generating an OFDM packet channel, the OFDM packet channel beingdivided into OFDM slots during which OFDM signals are transmitted withcapacity allocated on a per OFDM slot basis, the OFDM packet channel notbeing scrambled by the common scrambling code; combining the codeseparated CDMA channel(s) and the OFDM channel into a combined signaland transmitting the combined signal.

In some embodiments, the at least one code separated CDMA channel(s)comprises at least one continuous CDMA channels and a CDMA packetchannel that is divided into CDMA slots during which CMDA signals aretransmitted with capacity allocated on a per CDMA slot basis.

In some embodiments, the at least one code separated CDMA channel(s)comprises a CDMA packet channel that is divided into CDMA slots duringwhich CMDA signals are transmitted with capacity allocated on a per CDMAslot basis.

In some embodiments, the method further comprises for at least some ofthe OFDM slots, partitioning OFDM sub-carriers transmitted during atleast some of the OFDM slots between a plurality of receivers on apartitioning period basis.

In some embodiments, the method further comprises performing sub-carrierand power allocation dynamically as a function of mobility and/orfrequency selectivity.

In some embodiments, the method is adapted for use in a multiple carrierCDMA system, the method further comprising generating a a single setOFDM sub-carriers that spans the multiple carriers, and in which asingle wideband IFFT is performed to do OFDM modulation.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the attached drawings in which:

FIG. 1 shows the conventional WCDMA downlink structure;

FIG. 2 shows a new downlink structure provided by an embodiment of theinvention featuring an OFDM channel;

FIG. 3 is a slot structure for the OFDM channel of FIG. 2, as providedby an embodiment of the invention;

FIG. 4 is a flow chart showing a method of coding of the HS-SCCHinformation, as provided by an embodiment of the invention;

FIG. 5 is another example of an OFDM slot structure provided by anembodiment of the invention;

FIG. 6 is an illustration of the downlink structure of FIG. 2 inoperation;

FIG. 7 is a flow chart of an example method of scheduling OFDM and WCDMAusers on the downlink;

FIG. 8 is a block diagram of WCDMA mode reception in the presence ofOFDM-HSDPA;

FIG. 9 is a block diagram of OFDM-HSDPA mode reception in the presenceof WCDMA;

FIG. 10 illustrates prefix insertion and removal in accordance with anembodiment of the invention;

FIG. 11 shows an example of OFDM mode transmission and scheduling;

FIG. 12 is a block diagram of OFDM channel estimation in accordance withan embodiment of the invention;

FIG. 13 is a block diagram of WCDMA channel estimation;

FIG. 14 shows the conventional CDMA2000 release A\B\C channel structure;

FIG. 15 is a new channel structure featuring an OFDM mode provided by anembodiment of the invention;

FIG. 16 illustrates a method of reuse of the F-SPDCCH channel for OFDMtime-frequency plane allocation in accordance with an embodiment of theinvention;

FIG. 17 shows an example of how sub-carrier power allocation can beperformed based on the user channel condition for F-OFDM-PDCH;

FIG. 18 illustrates a preferred method of mapping the F-OFDM-PDCH onto amulti-carrier environment;

FIG. 19 is an example of an F-OFDM-PDCH structure provided by anembodiment of the invention;

FIG. 20 shows a block diagram of CDMA mode reception in the presence ofF-OFDM-PDCH; and

FIG. 21 shows a block diagram of F-OFDM-PDCH mode reception in thepresence of WCDMA signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2, shown is a downlink structure provided by anembodiment of the invention. Shown are the same channels discussedpreviously with reference to FIG. 1. In addition, there is an extra OFDM(orthogonal frequency division multiplexing) HSDPA channel 30. Thischannel is overlaid on top of the other channels using the same timeslot structure that is used to allocate the WCDMA HSDPA 18. In theillustrated example, this consists of two millisecond time slots. Thesetime slots are allocated using scheduling. A detailed example of amethod of user scheduling is provided below. This results in a downlinkslot structure having an OFDM space 32, a spreading factor equal 16 OVSF(orthogonal variable spreading factor) space 34, a large spreadingfactor OVSF space 36, and a synchronization code space 38 (moregenerally a signalling code space over which signalling channels can betransmitted that may or may not be for synchronization purposes). Thesynchronization channels are typically spread by a system-wide spreadingcode, and are combined at 40; the large spreading factor OVSF signals 36and the spreading factor equals 16 OVSF signals 34 are combined at 42and scrambled by a common scrambling code at 44. Then, the combined CDMAsignal produced as a result of the common scrambling code 44 and thecombined synchronization channels are combined at 46 with the OFDM HSDPAchannel 30 to produce the overall time domain output signal 47. In oneembodiment, in a given slot only OFDM HSDPA 18 or WCDMA HSDPA 30 istransmitted, as shown in the first two millisecond slot 26 of theexample shown in FIG. 2 in which case the slot is an OFDM slot or a CDMAslot respectively. In another embodiment, for some slots a WCDMA HSDPAsignal and a OFDM HSDPA signal are allowed to be transmittedsimultaneously during a given slot, for example is shown in the thirdtwo millisecond slot 27 of the example of FIG. 2. Here one slot containsboth an OFDM slot and a CDMA slot. These might be transmitter with lowerpower when both are present. In yet another embodiment, the WCDMA-HSDPA18 is removed entirely. The interference caused by the CFDM HSDPA 30 andseen by the other CDMA channels 10, 12, 14, 16 may be less than thatcaused by the low spreading factor WCDMA HSDPA 18.

As indicated above, the existing channels are scrambled using a commonscrambling code. However, the OFDM HSDPA channel is not scrambled by thescrambling code.

It is noted that the data rate for the OFDM slots in some embodiments isabout two times the data rate which would be possible on the WCDMA HSDPAslot.

It is noted that for WCDMA, the interference between the WCDMA HSDPAchannel and the other CDMA channels, for example the pilot channel andthe dedicated channel are well understood. Advantageously, the fact thatall of these channels are covered with a common PN code makes thesechannels appear as noise to the OFDM slot. Furthermore, the fact that ahigh spreading factor is used for these channels, for example 64, meansthat there is a high processing gain and the OFDM slots appear as noisefar below that of these channels taking into account the processinggain. For a 64 spreading factor, there is a 18 dB processing gain. Forthis reason, the transmission of the OFDM slots is substantiallytransparent to the existing CDMA transmissions.

Advantageously, this scheme is backwards compatible with existing UMTSsystems, and is also backwards compatible with existing WCDMA systems.New implementations will simply need to include the one additional mode,namely the OFDM mode which allows the transmission of OFDM slotsoverlaid over the existing transmissions. In some embodiments, the modeis dynamic as a function of time, and the slots during which OFDM isused as opposed to WCDMA on the HSDPA are determined on an ongoing basisby scheduling.

Referring now to FIG. 3, shown is a first example of an OFDM slotstructure that can be employed with the DL structure discussed withreference to FIG. 2. This slot structure has 2560 chips 49 for a 10/15ms slot which is identical to the number of chips per 10/15 ms slot inthe CDMA signal. Because of this, the summer 46 performs a singlechip-by-chip summing of the scrambled CDMA component and the OFDM slotstructure of FIG. 3. Advantageously, by tying together the slotstructure for the CDMA and CFDM components, some of the functionsperformed from CDMA such as synchronization and channel estimation, canbe re-used directly from the OFDM component. In the illustrated example,the data period 52 contains 18 IFFTs, each 128 chips in length, arrangedsequentially in time. With this structure, there is a 128 chip prefix 50which contains a designed training sequence, followed by a 2304 chipdata period 52, which is followed by a 128 chip suffix 56. The OFDMprefix contains known or highly reliable data and is provided to enhancethe channel estimation both for the OFDM capable mobile terminal and forthe legacy mobile. Preferably, a small size IFFT is employed to supporthigh speed mobility.

The example of FIG. 3 is a very specific slot structure that mountsnicely to the 2560 chip legacy slot structure. In a particular example,there is a prefix, a suffix and there is a data segment with 18 IFFTs.More generally, the number of IFFTs, the size of each IFFT and thepresence or absence of a prefix or suffix, and the size and content ofthe prefix and suffix, are all design parameters of a particularimplementation. Several detailed examples are provided herein, but it isnot the intent to limit the invention to these examples.

In some embodiments, the suffix is used to transmit a supplementaryHS-SCCH (High Speed Shared Control Channel). An example method of codingof the supplementary HS-SCCH is shown in FIG. 4, this also showingexample data transmitted on this channel. In this example, thesupplementary HS-SCCH is used to transmit information that allowsreceivers to determine where their content has been transmitted in thedownlink structure. This example provides a very specific method oftransmitting channelisation code set 163, modulation 164, R, S, Binformation 179, TSB, HARQ, NDI (new data indicator) and RV information181, and user equipment identifiers 165 using the suffix portion of theslot structure of FIG. 3. It is to be understood that these particulartypes of control information may not all be required for a givenimplementation, and the number of bits for each parameter may bedifferent for a given implementation. Furthermore, while it is preferredthis information is transmitted in the suffix of the OFDM slot structureof FIG. 3, more generally, it can be transmitted on any appropriatecontrol channel. In the illustrated example, the channelisation code set163 and modulation 164 are multiplexed with multiplexer 170 to produceeight bits 171 that are input to a one-half rate convolutional codingstep 172. This produces seventy-two bits 173 that are input to the ratematching function 174. The rate matching function 174 produces fortybits 175 that are input to a UE-specific masking function 176. Thisfunction takes the sixteen bit UE identifier 165 and performs a maskingof the output of the rate matching function 174 to produce a forty bitoutput 177.

The R, S and B outputs 179 are input to an RV coding function 118. Threebits output by the RV coding function 180, together with a six bit TBS,a three bit HARQ and an NDI bit, collectively indicated at 181, areinput to a multiplexer 182 which produces a thirteen bit output 183. Theeight bit output by multiplexer 170 and the thirteen bits output bymultiplexer 182 are input to a UE specific CRC function 184. The UEspecific CRC is calculated over the combination of the eight bits 171and the thirteen bits 183, and produces a sixteen bit CRC output 185.This is multiplexed with the thirteen bits 183 in multiplexer 186 toproduce a twenty-nine bit output 187. This output is subject toconvolutional coding at rate one-half as indicated at 188. This producesat seventy-four bit output 189 which is input to a rate-matchingfunction 190 which then produces an eighty bit output 191.

The output 177 contains the channelisation code set and modulationinformation. The eighty bits output at 191 contain the TBS, HARQprocess, RV information, and the new data indicator. The output 191 alsocontains the UE specific CRC calculated for all of the information,namely the channelisation code set, the modulation, the TBS, the HARQ,the RV, and the NDI.

The overall output contains information for three slots 192, 194, 196.The slot structure of FIG. 3 is repeated three times to transmit oneHS-SCCH sub-frame 197. The first third of the sub-frame 192 istransmitted during the first slot. The second portion 194 of thesub-frame is transmitted during the second. The third portion 196 of thesub-frame is transmitted during the third slot. The sub-frame structurethen repeats itself. It can be seen that forty bits are transmittedduring each 128 chip suffix.

The sixteen bit UE identifier identifies which receiver is to receivethe current HSDPA content.

In another embodiment, the OFDM slot structure shown in FIG. 5 isemployed. With this structure, there is a 128 chip prefix 66 followed by1024 chip data portion 68 followed by a 128 chip suffix 70 which combineto produce a 1280 chip, 5/15 ms sub-slot 62. In this case, the dataportion 68 consists of sixteen 64 chip FFTs transmitted in sequence.This is then repeated with a separate sub-slot 64 to generate an overall2560 chip slot 60. The suffixes may for example be used to send systeminformation or to broadcast or to send short messages. The prefixes inthis embodiment are again used to transmit designed training sequenceswhich can be employed for channel estimation. In some embodiments, slotsare 0.67 ms in duration, but are allocated (scheduled) in groups ofthree, or 2 ms periods, also referred to as TTIs (transmit timeintervals), this being 3GPP terminology. In some embodiments, the groupsof three slots do not need to be aligned with the TTIs in the remainderof the transmission. An example of this is shown in FIG. 10 where thestart of the three slots for the OFDM is two slots delayed from thestart of the TTI. However, the timing of the OFDM slots is still tied tothe timing of the CDMA signals.

Preferably the HS-SCCH is used to indicate to a given user whether ornot it is scheduled to receive OFDM data during a given slot. FIG. 6shows an example of how the downlink operation can proceed. Conventionalusers indicated generally at 28 will already demodulate the HS-SCCH 19.Every WCDMA user 16 equipment 28 demodulates the common channels 14,HS-SCCH 19 and the DCH 30 all the time. Then, the WCDMA UE 28demodulates the WCDMA HSDPA 30 channel if it is scheduled. Thesupplementary HS-SCCH described above with reference to FIGS. 4 and 5 isanother control channel which is available for transmitting to users. Insome embodiments this is transmitted using CDMA technology, but duringthe suffix of the OFDM time slot. In some embodiments, the existingHS-SCCH is used to tell a user whether or not to look at thesupplementary HS-SCCH.

Each OFDM/WCDMA UE generally indicated at 29 demodulates the commonchannels 14, the HS-SCCH 19 and DCH 16 all the time. The OFDM/WCDMA UE29 demodulates OFDM HSDPA channel 18 only if it is scheduled.Preferably, during the prefix period and/or the suffix period, nothingis transmitted on the OFDM channel 30. When this is done, the receivercan clearly see the remaining channels and in particular can see theCPICH if transmitted during these periods. Then, the CPICH that istransmitted by the conventional CDMA transmitter can be received andtreated as a prefix for the purpose of the OFDM channel reception. Toachieve this, zeros are placed there for the OFDM transmission.Alternatively, something else that can be accurately demodulated couldbe placed there such as a pre-defined training sequence and/or aclassical OFDM cyclic pre-fix.

Another embodiment of the invention provides a method of scheduling thedown link introduced above.

FIG. 7 shows an example of the scheduling method. In step 7-1, allWCDMA-only UEs measure a channel quality indicator in respect of WCDMAand feed these back to a node responsible for scheduling, hereafterreferred to as a scheduler, for MCS scheduling.

In step 7-2, the OFDM capable user equipment (UE) measures the channeldispersion and mobility and determines a channel quality indicator foreach of WCDMA and OFDM and performs a comparison. More particularly, acomparison is made between the channel quality indicator for WCDMA,referred to as CQI_(-WCDMA) and the channel quality indicator for OFDM,referred to as CQI_(-OFDM) is made. If CQI_(-WCDMA) is greater than orequal to CQI_(-OFDM), then only the CQI_(-WCDMA) is fed back to thescheduler. In this case, the OFDM capable UE uses a simple rake receiverto modulate the HSDPA channel. On the other hand, if CQI_(-WCDMA) isless than CQI_(-OFDM) then only CQI_(-OFDM) is fed back, and the OFDMcapable UE uses OFDM mode to demodulate the HSDPA channel. Then, at step7-3, the scheduling node schedules OFDM-HSDPA and WCDMA-HSDPA usersbased on these two quality measurements respectively.

Assuming proper power allocation the OVSF code can be guaranteed toco-exist with OFDM. To demodulate the common channels and the dedicatedchannel the steps of de-scrambling, de-spreading, and soft de-mappingand FEC decoding are performed. After the step of de-spreading, the OFDMsignal portion will be converted into AWGN. In other words the fastHadamard transform of an IFFT can be approximated by additive whiteGaussian noise. An example of a very specific co-exist condition is asfollows:P _(DCH)+10log₁₀ SF _(DCH) −P _(OFDM) −P _(COMMON)>DECODE_(target)(dB)

The above equation, P_(DCH) is the power of the DCH channel, SF_(DCH) isa spreading factor of that channel, P_(OFDM) is the power of the OFDMchannel, P_(COMMON) is the power of the common channels, andDECODE_(target) is a minimum SNR requirement for a common channel (sayvoice) or a dedicated channel. This ensures backward compatibility withlegacy UE.

CDMA Reception

Advantageously, in some embodiments changes may not be required to theCDMA RAKE receiver.

In some embodiments, interference cancellation is performed prior todemodulating the OFDM slots so as to remove the effects of the CDMAsignal. This can be done by performing conventional rake reception onthe CDMA signal to generate a channel estimate. The common channel,voice channel, etc are de-spread, for example using a fast Hadamardtransform (FHT) then, hard decision detection is performed for all ofthese channels, and the channels are then re-generated using the harddecisions convolved with the time domain channel response. This thengives a time domain representation of the interference due to thesechannels. This time domain representation is then subtracted from theinput to remove the CDMA interference from the OFDM portion.

Advantageously, much of this processing will already have been done inconventional, CDMA receivers, and as such this can be reused.

Once the interference due to the CDMA portion has been subtracted, theOFDM slots can be demodulated, and any OFDM demodulation approach can beused. Preferably the conventional OFDM demodulation approaches areemployed. It is noted that the OFDM slots are sent in the same frequencyband and at the same time as existing CDMA transmissions.

FIG. 8 is a block diagram of WCDMA mode reception of a signal 80containing the WCDMA content in the presence of OFDM-HSDPA. Preferably,completely conventional WCDMA reception is performed. This involvesperforming finger searching, assignment and channel estimation 22 in aconventional way for CDMA. The output of this is used to de-scramble andde-spread 90, 92, 94 all of the multi-path components (only three shown)and the output of this process is fed into an MRC (maximum ratiocombining) process 96 which produces an output 98 consisting of thereceived signal plus additive white Gaussian noise due to the OFDMcomponent.

FIG. 9 is a block diagram of a example receiver for the OFSM-HSDPAreception. A received signal 100 has a desired OFDM component and aWCDMA component which is interference. It can be seen that parts of thisreceiver form a standard WCDMA receiver 101, and this block contains thechannel estimation 102, rake receiver 104, soft de-mapping 106 and FECdecoding functions 110.

The output of the rake receiver 104 is also passed to a hard decisionblock 108 which generates hard decisions upon the output of the rakereceiver 104. These outputs are re-spread and re-scrambled 14 andfinally convolved with the channel impulse response 112 to get a goodtime domain estimate of the contribution to the received signal 100 dueto the WCDMA component. This signal is then subtracted with subtractor116 from the received signal 100 to produce a signal 117 which issubstantially only the OFDM component. This is fed through an OFDMchannel estimator 118 which produces an OFDM channel estimate 119, andprefix processor 120 which produces a prefix processor output 121. TheOFDM signal 117, the channel estimate 119 and the prefix processoroutput 121 are all fed into the FFT block 122 and OFDM reception is thenconducted to produce overall output 124.

Preferably, the prefix processor is an optional component as defined inapplicants co-pending application Ser. No. 10/662,465 filed Sep. 3,2003, hereby incorporated by reference in its entirety.

FIG. 10 shows an example of slot alignment for the OFDM HSDPA with theremainder of the transmission. Here, the start 125 of the OFDM sub-frame126 (one TTI in duration) is aligned with the start 128 of the thirdslot (slot #2) 29 in the regular TTI 127. Other alignments are possibleso long as there is a known relationship in the timing. Preferably, the“0” insertion is performed as discussed previously such that the pilotchannel has no OFDM interference.

FIG. 11 provides an example of OFDM mode transmission and scheduling.Four channels are shown schematically, namely the HS-SCCH 500, theHS-PDSCH (high speed packet downlink signalling channel) 502, theDL_DPCH (downlink packet channel) 504, and the UL_DPCCH (uplinkdedicated packet control channel). The HS-SCCH is used to transmitcontrol information in a 2 ms sub-frame. One such sub-frame isillustrated at 508. The HS-PDSCH 502 is transmitted slotwise as shown byway of example at 510. The DL_DPCH 504 is transmitted slotwise as shownby way of example at 514. Finally, the UL_DPCCH 506 is transmittedslotwise as shown by way of example at 518. The scheduling processbegins with the UE receiving control information on the HS-SCCH. Then,the UE receives a transport block on one or several HS-PDSCH. Next, theUE positions the HS-DPCCH with respect to the HS-PDSCH. In an exampleimplementation, the HS-DPCCH is positioned m×256 chips after theUL_DPCCH with m such that ACK\NACK starts within 0 to 255 chips after7.5 slots following the reception of the HS-PDCH sub-frame. Thus, inthis example the positioning of the HS-DPCCH is tied to both theUL_DPCCH and the HS-PDSCH.

FIG. 12 shows an example of how OFDM channel estimation can be employedwhich is based on the WCDMA pilot. WCDMA searching and correlation isperformed 13 to generate a time domain channel response 131 which isthen padded with zeros, and converted to frequency domain channelresponse 135 with an FFT function 134, converted to parallel form withserial to parallel converter (S/P) 132. The frequency domain channelresponse 135 is then filtered collectively, with filter UH 136 and theneach sub-carrier channel response is filtered with a respective filter.Only the filters 132, 140 from sub-carriers “1” and “K” are shown. Theseare filtered again with filter UH 142 to provide a new improvedfrequency domain channel response 143. This is then used for OFDMdemodulation. Passing this improved frequency domain channel responsethrough an IFFT function 144 results in an improved time domain channelresponse 145 which can be used for WCDMA demodulation.

The filters UH and pi are constructed to filter out the time profiles ofthe impulse channel response. For example, the real world channelresponse may only span L samples, so after L samples the remainingsamples are set to zero to reduce the noise.

FIG. 13 shows an example of how WCDMA channel estimation can be employedwhich is based on OFDM pilots. The OFDM symbols can be used to transmitpilot subcarriers, for example in a scattered fashion. This processbegins by converting a serial stream of samples to parallel form withS/P function 150, this being a “received FFT signal” 151. The receivedFFT signal 151 is converted to a time domain signal 153 with an FFTfunction 152. Then, the pilots are extracted, and the remainder of theprocess is concerned with extracting the channel using the pilots. Inother words, preferably only the OFDM pilots are used for channelestimation. This involves filtered the pilot channel estimatescollectively, with filter UH 154 and then filtering each sub-carrierchannel response individually with a respective filter, two such filters156, 158 being shown in the illustrated example. These are then filteredagain with filter UH 160 to provide a new improved frequency domainchannel response 161. This is then used for OFDM demodulation. Passingthis improved frequency domain channel response through an IFFT function162 results in a time domain channel response 163 which can be used forWCDMA channel and finger searching.

Hybrid Mode Assignment

Various mechanisms can be used to assign modes of operation in systemsin which both the OFDM-HSDPA and the WCDMA-HSDPA are available.Preferably, the common channel and DCH are set up and operation as perrelease 5, a UMTS standard release for HSPDA. Preferably, the HSDPAchannel (referring to both the OFDM and WCDMA components) is scheduledby a scheduler in a base station. The OFDM-HSDPA and WCDMA-HSDPA is adynamic parameter meaning the sequence of transmission of the OFDM-HSDPAand/or the WCDMA-HSPDA can be changed dynamically. The OFDM mode ispreferably flagged in the HS-SCCH channel. In a preferred embodiment,there is a TDM (Time Division Multiplexing) arrangement of OFDM-HSDPAand WCDMA-HSDPA modes. However, as described previously, these two modescan co-exist preferably with a reduction in the power used for each ofthe modes. The HARQ (Hybrid Automatic Repeat Request) re-transmissionmode follows the first transmission.

Voice and CBR-LDD conversational services are preferably mapped onto therelease 4 DCH channels for both WCDMA UE's and OFDM UE's. Backgrounddata download and streaming services are preferably mapped onto theHSDPA service.

It is noted that the OFDM transmissions can be mapped onto a MIMO(Multiple Input Multiple Output) configuration. In such an embodiment,preferably MIMO pilots are embedded within the OFDM transmissions. TheWCDMA channels are still mapped onto non-MIMO channels. This supportsOFDM transmit diversity and supports OFDM TxAA.

An example set of parameters that summarises the two specific OFDM slotstructures described above with respect to FIGS. 3 and 5 is provided intable 1 below. It is to be clearly understood that this is simply oneexample of an OFDM slot structure. The TTI duration may be different andmay contain a different number of slots. The FFT size may be different,and may contain a different number of smaller FFTs or be simply a singlelarge FFT. The sampling rate may be different from that described.However, preferably the sampling rate is identical to that used in theunderlying CDMA system. The ratio of the OFDM sampling to the UMTS chiprate is preferably the UMTS oversampling rate. The guard time of 256chips is but one example. Preferably, the guard time is a time spanlarger than the multi-path delay spread. The sub-carrier separation is afunction of the amount of bandwidth to be employed and the number ofsub-carriers used. The number of OFDM symbols per TTI is 18 for the FIG.3 embodiment, each symbol being a 128 point FFT, and is 16 for the FIG.5 embodiment, each symbol being a 64 point FFT. Other numbers mayalternatively be employed. The OFDM slot duration of the twoconfigurations maps nicely upon the slot structure of the underlyingCDMA systems. Preferably the slot structure is tied to the CDMA timingso that the CDMA timing can be used for the OFDM slots as well. The OFDMbandwidth is preferably similar to that used for the CDMA system. In amulti-carrier system, this OFDM bandwidth can increase. For theparticular set of features/parameters given, the peak channel rates inthe table are the result. However, these are clearly subject to theselection of all the other parameters. For the two examples, the totalsystem overhead (guard interval etc.) is less than ten percent. TABLE 1Parameters Config. A Config. B TTI duration 2 ms or 3 slots 2 ms or 3slots FFT size 2304 or 18 × 128 FFT 1024 or 16 × 64 Sampling rate 2 ×3.84 Msps or 2 × 3.84 Msps or exactly same as UMTS exactly same as UMTSRatio of OFDM UMTS over sampling UMTS over sampling sampling to UMTSrate rate chip rate Guard time 256 chips = 66.66 μs 256 chips = 66.66 μsSub-carrier 1.66666 kHz 3.75 kHz Separation # of OFDM symbols 1 2 perTTI OFDM symbol duration 0.667 ms 0.3333 ms # of useful sub- 2048 or TBD940 or TBD carriers OFDM bandwidth 3.84 MHz 3.84 MHz Peak channel rate 2× 12.288 Mbps - 2 × 11.28 Mbps with QAM16 coding overhead Total system<10% <10% overhead

FIG. 15 shows a preferred embodiment of a new OFDM mode for use in aforward packet data channel (F-PDHC). The embodiment is designed toallow bandwidth compatibility with 1xEV-DV, and as such a specific framestructure is proposed. However, the invention is not limited to theparticular frame structure, particularly if no compatibility isrequired. Enhancements and further embodiments relating to thisembodiment will be described with reference to FIGS. 16 to 21. Shown inFIG. 15 are the conventional release-A/B channels 200 introduced topreviously with reference to FIG. 14 which consist of a CDM pilot 204,common channels and traffic channels. Also shown is the new forwardpacket data channel 240 having dual modes, one of which is OFDM 242 andthe other one of which is CDMA 244. The forward packet data channel 240is used to transmit scheduled packet content. In the illustratedexample, the overall frame is 20 milliseconds in duration and contains16 slots each of which is 1.25 milliseconds in duration. Each slotcontains 1536 chips. As in the first set of embodiments, the OFDM frameis preferably mapped onto the CDMA signal such that the chip by chipsumming can be performed, and such that timing derived from the CDMAsignal can be used for the OFDMA signals as well. Other slot sizes andframe sizes can alternatively be employed. In one embodiment, each slotcan be scheduled to be either transmitting the packet data channel inCDMA mode or in OFDM mode, but not in both. In another embodiment, bothCDMA and OFDM mode transmission is allowed simultaneously during asingle slot. In yet another embodiment, there is no CDMA for the forwardpacket data channel.

A single long PN code (not shown) is provided which covers all of theCDMA channels including the standard release-A/B channels 200 and theforward packet data channel in CDMA mode 244. There is no long PN codewhich is used to cover the OFDM channel 240. Also shown is an F-SPDCCH(forward secondary packet data control channel) channel 249 which is acontrol channel that is used to schedule the CDMA packet channel 244.Preferably, a flag is provided to indicate for a given slot that OFDMmode is being used. Other than this flag, the structure of this channelpreferably is the same as the existing F-SPDCCH provided for in the CDMAsystems (channel 218 of FIG. 14).

In some embodiments, the OFDM time-frequency plane is allocabledynamically to one or more users/receivers during a given slot. Anexample of this is illustrated in FIG. 16 where 9 1.25 millisecondsslots are shown, these consisting of a first group 260 of two slots, asecond group 262 of two slots, a group 264 of five slots, and a finalslot 266. During each group of slots, the sub-carriers used for OFDMmode forward packet data channel are allocatable to one or two users.Other numbers of users could be supported. The particular users forwhich capacity is to allocated are identified by MAC-ID's, one for eachuser. In the illustrated example, four different MAC-ID's are employed,namely, MAC-ID=1, 2, 3, 4. The F-SPDCCH contains two sub-channels 270,272 for identifying the users to be scheduled during a given slot. Thefirst sub-channel 268, namely F-SPDCCH(0) identifies a MAC-ID of a firstuser. The second sub-channel 270, namely F-SDPCCH(1) identifies theMAC-ID of a second user when present. When there is no second user,preferably the second sub-channel 270 is not transmitted to savetransmit power. In the illustrated example, during the first two slotsthe first sub-channel indicates MAC-ID=1 and the second sub-channelindicates MAC-ID=2. Furthermore, for each MAC-ID there is a fieldindicated to be “LWCI” which stands for Last Walsh Code Indicator. Thisis taken from the F-SPDCCH channel usage for CDMA. For OFDM applicationshowever, this is preferably used to indicate the last sub-carrier indexfor the particular user. Thus, the available sub-carriers can bedynamically assigned between two users, and moreover the particularsub-carriers to be assigned can also be dynamically determined. Ofcourse, it can easily be seen how this approach can be extended tohandle the data of more than two users during a given slot.

In the illustrated example, the F-PDCH has 11 sub-carriers (this beingarbitrary, only for the purpose of illustration) having sub-carrierindex 272 from 1 to 11.

For the first two slots 260 of FIG. 5, the first 5 sub-carriers are usedfor MAC-ID=1 and the next 6 sub-carriers are used for MAC-ID=2.

Continuing with the example, during the next two slots 262 MAC-ID=1identifies the first user, and that user is given the first sevensub-carriers. MAC-ID=3 identifies the second user and that user is giventhe last four sub-carriers.

During the next four slots 264, only a single user is transmitting,namely the user with MAC-ID=1. During the last slot 266, MAC-ID=4 andMAC-ID=2 identify two users.

Preferably, the F-SPDCCH sub-channels are spread and transmitted as CDMAchannels. In this example, each sub-channel transmits a respectiveMAC-ID and a respective last Walsh code indicator in each slot. This isa very specific method of conveying this control information toreceivers. More generally, when sub-carrier allocation is to beemployed, any appropriate method of instructing the receivers to look inparticular slots and at specific sub-carriers can be employed. Theillustrated example has assumed only two users can be scheduled perslot, and that there are eleven sub-carriers to use. More generally, anyappropriate number of sub-carriers can be used, and any appropriatenumber of users can be scheduled. Particular numbers of sub-carriers maybe more amenable to being overlaid over the CDMA signals than others.

It is noted that in the above where is it mentioned the user istransmitting, what is meant is that data for that user is beingtransmitted over the shared channel from a network transmitter such as abase station.

In a preferred embodiment, the new forward OFDM packet data channelallows sub-carrier power allocation based on user channel conditions.Referring to FIG. 17, shown in the right part of the figure is amobility-frequency-selectivity chart 311 which maps frequencyselectivity 310 and mobility 312 to preferred operating conditions. Forhigh frequency selectivity and mobility, preferably the OFDM packet datachannel is used, and a narrow band is employed with high power. Thedotted line 324 represents the transition between conditions where OFDMis preferred and where CDMA is preferred. In the lower left hand cornerabove the dotted line 324, where frequency selectivity is lower andmobility is lower, it is preferred still that OFDM is employed, but witha wide band and low power. For users outside the two particular ranges,an intermediate band and an intermediate power are preferably employed.

The left part of FIG. 17 shows an example of sub-carrier and powerallocation 301. One axis 302 is for sub-carrier index, and the otheraxis 300 is for time. The radial axis (not shown) is for power. Anexample of sub-carrier and power allocation for the users, user-1,user-2 and user-3 is shown. User-1 is shown to have a position 326 inthe mobility-frequency selectivity chart 311 indicating high mobilitybut relatively low frequency selectivity. For this user, a wide bandmedium power transmission is preferably used as indicated by thesub-carrier allocation 304 in the sub-carrier power allocation 301. Thisshows that four different sub-carriers are used to transmit during fourdifferent OFDM symbols. For the second user, user-2 which is operatingwith both low mobility and low frequency selectively but not so low thatCDMA is preferred as indicated by position 328 on the mobility-frequencyselectivity chart 311, the sub-carrier and power allocation 301 showssub-carriers 306 for this user consisting of three or four differentsub-carriers are used but over a longer period of time and with lowerpower. The number of sub-carriers allocated to user-2 changes from threeto four after the fourth OFDM symbol in the example. Finally, for thethird user, user-3 who has both high frequency selectivity and highmobility as indicated by position 330 in the mobility-frequencyselectivity chart 311, a sub-carrier power allocation 308 of only twosub-carriers is employed, but with a high power and as such a narrowband high power signal is being transmitted.

Preferably, there will be a total power budget for the OFDM packet datachannel, and the total power can be manipulated between the sub-carriersto provide the best operating conditions for all users. It is noted thatsome mechanism for determining a metric representative of mobility andfrequency selectivity at the base station may be needed for thisembodiment. In one embodiment, each mobile terminal measures this andfeeds one or more parameters back to the base station.

In another embodiment, the forward OFDM packet data channel is adaptedfor use in a multi-carrier CDMA system. In a preferred embodiment,rather than computing a respective IFFT for each of a respective OFDMsignal to be transmitted on each carrier, a single wide band IFFT isused to extract and modulate all sub-carriers at both the transmitterand the receiver. This is best illustrated by way of example, and oneexample is shown in FIG. 18. In this example, there is a multi-carrierCDMA system having three carriers 342, 344, 346 labelled carrier #1,carrier #2 and carrier #3. A large set 347 of OFDM sub-carriers is shownto span the bandwidth of all three carriers. In the illustrated example,this set 347 is shown to include 26 sub-carriers, which span all threecarriers. It is to be understood that different numbers of carriers anddifferent numbers of sub-carriers can be employed within the scope ofthe invention. Preferably, the OFDM system is designed to perform asingle IFFT function at the transmitter for all 26 sub-carriers.Similarly, at the receiver the OFDM system is designed to perform asingle FFT function for all 26 sub-carriers. Preferably, one or moreidle carriers are inserted to provide a guard band between the carriers.In the illustrated example, OFDM symbol 348 has actual data on 6sub-carriers in the band of carrier #1, 6 sub-carriers in the band ofcarrier #2, and data on 6 sub-carriers in the band of carrier #3. Twosub-carrier guard bands 350 are shown as well. In the illustratedexample, 1.25 ms slots contain 5 OFDM symbols each of which is a 26point FFT. A specific number of symbols per slot, carriers andsub-carriers per carrier has been shown in the illustrated example. Moregenerally, it can be seen how a single IFFT can be used to modulate somenumber of sub-carriers to some number of carriers.

In some embodiments, the OFDM and CDMA modes are assignable on a percarrier basis. Then, if a given carrier is to be used to transmit CDMArather than OFDM, all that needs to be done is to insert zeros in theOFDM signal for the sub-carriers which are representative of thatcarrier. FIG. 18 also shows an example of this. In the illustratedexample, three 1.25 millisecond slots are shown. During the first slot347 (the bottom most slot) carrier #1 is being use to transmit CDMA, theCDMA spectrum being indicated at 352. Carrier #2 and carrier #3 are usedto transit OFDM, and moreover the sub-carriers are allocated betweenthree different OFDM users. In the illustrated example, there are alsoguard bands between the sub-carriers used for different carriers. Thisis realized by inserting zeros at the input to the IFFT function. Thus,to perform the IFFT for the first slot 347, zeros would be inserted forall the sub-carriers associated with carrier #1, and for the guardbands, the data for the first user is inserted for the six sub-carriersof carrier #2, data for the second user is inserted for two of the sixsub-carriers of carrier #3, and data for a third user is transmitted onfour sub-carriers of carrier #3. Preferably, all of this is dynamicallyassignable on a per-slot basis. For example, in the second slot 345 (themiddle slot of FIG. 18) all three carriers are used for OFDM. In theillustrated example, the 6 sub-carriers of the first carrier are usedfor a first user, four sub-carriers of the second carrier and foursub-carriers of the third carrier are used for a second user, twosub-carriers of the second carrier are used for a third user and twosub-carriers of the third carrier are used for a forth user.

Preferably, for single or multiple carrier systems, the OFDMsub-carriers are partitioned between multiple users every partitioningperiod. A partitioning period may be a single OFDM symbol, or may be alonger period.

Furthermore, as indicated above, for multiple carrier systems preferablyeach carrier is allocated to only one of CDMA or OFDM, and this is doneon a per allocation period basis, and for any carrier that is allocatedto CDMA during a given allocation period, zeros are inserted in aportion of the OFDM IFFT that overlaps the carrier during thatallocation period. The allocation period can be any appropriate time andmay be static or variable, a single slot, or some longer time.

An example slot structure for the OFDM packet data channel is shown inFIG. 19. In this case, a 1.25 milliseconds slot 360 (1536 chips andduration) begins with a 256 chip OFDM SPDCCH 362 followed by 8 OFDM 128chip FFT's 364, 366, 368, 370, 372, 374, 376, 378 between each of whichthere is a 22 chip “0” insertion that functions as a prefix. Preferably,the F-OFDM SPDCCH is used to indicate which users are scheduled for OFDMtransmission, and which sub-carriers, as discussed in detail above. The22-chip insertion is used for the prefix, as is well known for OFDM.Alternatively, low rate reliable data can be transmitted during theseperiods. The use of the prefix allows the removal of inter-symbolinterference.

It is to be understood that the OFDM signals will act as interference tothe existing CDMA mode reception. An example of CDMA mode reception inthe presence of the F-OFDM Packet data channel is illustrated in FIG.20. The CDMA plus OFDM signals are input to the de-scrambling functions402,404, 406 (one for each multi-part component), which then de-spreadswith the fast Hadamard transforms 402, 410, 412. Finger searchsynchronization and channel estimation 416 is used to find themulti-path components to be de-scrambled. Then, maximum ratio combiningis performed. The de-scrambling, FHT and MRC are all conventional rakereceiver functions. The output 420 of this process is the desired signalplus additive white Gaussian. This is because the OFDM signal was notscrambled using the PN code that was used in scrambling the remainder ofthe signal, and as such when the input signal is de-scrambled, theeffects of the OFDM signal become noise.

It is noted that in the example implementations the spreading gainintroduced by the CDMA system will provide a 15 DB gain over the powerlevel of the OFDM signal, and as such the OFDM signal is negligible.More generally, the spreading gain of the CDMA signal and the powerlevel of the OFDM signal should be considered together to ensureacceptable performance for the CDMA signals.

Similarly, the CDMA signals will act as interference to the OFDM signalswhen they are received. An example implementation for forward OFDMPacket data channel mode reception is shown in FIG. 21. For thisembodiment, the signal 430, again containing OFDM and CDMA is input intothe receiver. Part of the receiver is a conventional CDMA receiver whichperforms channel estimation 432 in the time domain and rake reception434 followed by soft de-mapping 436 and Forward Error Correctiondecoding 440. The output of the Rake receiver 439 is also fed to a harddecision block 438 which makes hard decisions on the received datawithout decoding them. These hard decisions are then re-spread andre-scrambled 444. This is then convolved 442 with the channel responseto give a time domain estimate 443 of the contribution to the receivedsignal due to the CDMA component. This estimate 443 is then subtractedfrom the overall received signal 430 to give an estimated OFDM component447, which is then demodulated using conventional OFDM receptiontechniques 452. Thus, a form of interference cancellation is employed toremove the effects of the CDMA from the OFDM signals at the receiver.

Another embodiment provides a method of controlling the power control ofCDMA such that the interference introduced by the OFDM channel will notresult in “power control panic”. When an OFDM slot is scheduled, this isnot orthogonal to the existing CDMA signals. It is noted that if thecondition is flat fading, it is preferred that rather than transmittingOFDM, CDMA is sent. This is because in a flat fading condition, CDMAexhibits the same performance as OFDM, but has the benefit of also beingorthogonal with the existing CDMA channel. Thus, in sufficiently flatfading conditions, packet data is preferably sent using the CDMA mode.

On the other hand, if a channel is dispersive, the orthogonality betweenthe different channels of CDMA including CDMA packet data channel willbe lost. Thus, the expected orthogonality will not result in theexpected interference reduction. In such a channel, OFDM is good. Itfixes the bad channel, but will also introduce some degree ofinterference with existing CDMA systems. This interference will bebursty in nature. This will have two impacts upon the existing CDMAchannels. There will not be much of an effect on the common channelsbecause there is a large spreading factor employed for these channels.However, it will effect the power control channels such as the voicechannels. This may cause power control loop panic as the voice channelstry to increase their transmit power to overcome the interferenceintroduced by the OFDM. According to a preferred embodiment of theinvention, the transmitter fixes the power control panic by changing thepower control parameters to avoid this power control panic ahead oftime. In one embodiment, this simply involves changing the outer looppower control thresholds ahead of the transmission of OFDM packet datatransmissions.

It is noted that for current release A/B/C, Walsh space exhaustionresults in a fundamental limit upon capacity. According to a preferredembodiment, the forward OFDM packet data channel can be treated as asolution for Walsh space extension. In this case, the packet data ismapped into Fourier space. Thus, the Walsh code limit is changed by theintroduction of the additional Fourier space in the same transmitbandwidth. Effectively, additional dimensions are provided even afterthe Walsh space has been exhausted.

A preferred service partition is to map a Walsh space onto all releaseA/B Channels and to use the Fourier space for the forward packet datachannel.

As discussed above, in a preferred embodiment, an intelligent decisionis made as to which user and which sub-carriers are to be transmitted inOFDM mode. In an example method making such an intelligent decision, aCDMA correlator is first used to search rake fingers and to measure thechannel impulse response. The FFT of a channel impulse response istaken, and a first through nth order moments of this FFT are alsocomputed. Based on these moments, the frequency-selectivity and mobilityof the channel can be determined. As a function of thefrequency-selectivity and mobility, the sub-carrier and power allocationare performed. Channel quality indicator for CDMA and OFDMA may also becomputed. The channel quality indicators will determine whether or notOFDM mode or CDMA mode is to be scheduled, and the sub-carrier and powerallocation is performed in the event that OFDM is scheduled. Othermethods of deciding which of OFDM or CDMA may be employed.

Finally, it is noted that for MIMO applications, it is preferred thatOFDM is employed for a MIMO based forward packet data channel. The CDMAchannels can be mapped onto non-MIMO channels.

In another embodiment, a combined CDMA/OFDM downlink structure isprovided that is data only (DO). This embodiment can be described withreference to the drawings used for the non-data only embodiments. In aDO embodiment corresponding to the FIG. 2 WCDMA-based embodiment, thereis no DCH 16. In a DO embodiment corresponding to the FIG. 15CDMA-2000-based embodiment, there are no traffic channels.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

1. A transmitter adapted to transmit a downlink signal comprising: at least one code separated CDMA (code division multiple access) channel(s) that are scrambled with a common scrambling code; an OFDM packet channel overlaid over the CMDA channels, the OFDM packet channel being divided into OFDM slots during which OFDM signals are transmitted with capacity allocated on a per OFDM slot basis, the OFDM packet channel not being scrambled by the common scrambling code.
 2. A transmitter according to claim 1 wherein the at least one code separated CDMA channel(s) comprises at least one continuous CDMA channels and a CDMA packet channel that is divided into CDMA slots during which CMDA signals are transmitted with capacity allocated on a per CDMA slot basis.
 3. A transmitter according to claim 1 wherein the at least one code separated CDMA channel(s) comprises a CDMA packet channel that is divided into CDMA slots during which CMDA signals are transmitted with capacity allocated on a per CDMA slot basis.
 4. The transmitter according to claim 2 adapted to transmit packet data on a sequence of slots, each slot being either an OFDM slot or a CDMA slot, but not both.
 5. The transmitter according to claim 2 adapted to transmit packet data on a sequence of slots, each slot being an OFDM slot and/or a CDMA slot.
 6. The transmitter according to claim 1 further adapted to transmit periodically at least one CDMA signalling channel that is not spread by the scrambling code.
 7. The transmitter according to claim 6 wherein during periods of overlap between the transmission of the at least one signalling channel and the transmission of an OFDM slot, the OFDM slot transmits zeros or known data so that those periods can be treated as a prefix for the OFDM slots upon reception.
 8. The transmitter according to claim 1 wherein each OFDM slot comprises: a prefix followed by an IFFT period, followed by a suffix.
 9. The transmitter according to claim 8 wherein each OFDM slot is 10/15 ms in duration, and wherein the prefix is 128 chips, the IFFT period is 2304 chips, and the suffix is 128 chips.
 10. The transmitter according to claim 8 wherein the IFFT period comprises: a plurality of IFFTs separated by zero insertions.
 11. The transmitter according to claim 10 wherein the IFFT period comprises 18 128-chip IFFTs.
 12. The transmitter according to claim 8 wherein the prefix is a designed training sequence.
 13. The transmitter according to claim 6 wherein the at least one signalling channel is transmitted during the prefix of the OFDM slots, and wherein the OFDM slots transmit zeros during the prefix so that the OFDM channel does not interfere with the at least one signalling channel.
 14. The transmitter according to claim 2 further adapted to transmit a control channel that identifies to receivers which OFDM slots and/or which CDMA slots are for a given receiver.
 15. The transmitter according to claim 8 adapted to transmit a control channel during the suffix that identifies to receivers which OFDM slots and/or which CDMA slots are for a given receiver.
 16. The transmitter according to claim 11 wherein the control channel contains a receiver specific mask and/or a receiver specific CRC code.
 17. The transmitter according to claim 1 wherein each OFDM slot comprises in sequence: a first prefix; a first IFFT period, a first suffix; a second prefix, a second IFFT period, and a second suffix.
 18. The transmitter according to claim 17 wherein each OFDM slot is 10/15 ms in duration, and each prefix is 128 chips, each IFFT period comprises 16 64 chip IFFTs, and each suffix is 128 chips.
 19. A system comprising the transmitter of claim 1 in combination with a plurality of receivers at least one of which is an OFDM-capable receiver, wherein each OFDM-capable receiver demodulates the CDMA channels on an ongoing basis, and each OFDM-capable receiver demodulates a given OFDM slot only if scheduled during the OFDM slot.
 20. A system comprising the transmitter of claim 2 in combination with a plurality of receivers at least one of which is an OFDM-capable receiver, and each OFDM-capable receiver demodulates a given OFDM slot only if scheduled during the OFDM slot, and each receiver demodulates a given CDMA slot only if scheduled during the CDMA slot.
 21. The transmitter of claim 1 further comprising a scheduler adapted to schedule receivers on the downlink by: for each of a plurality of OFDM-capable receivers, obtaining a respective channel quality indicator for CDMA slots and/or for OFDM slots; scheduling transmission to OFDM-capable receivers on CDMA and/or OFDM slots according to the channel quality indicators.
 22. The transmitter of claim 21 wherein for a given receiver, the channel quality indicator for CDMA slots and/or for OFDM slots comprises a better of the CDMA channel quality indicator and the OFDM channel quality indicator as determined by the receiver.
 23. The transmitter of claim 15 wherein the scheduler is further adapted to: for CDMA-only capable receivers, obtain a channel quality indicator for the CDMA slots only, and schedule transmission to CDMA-only capable receivers on the basis of the CDMA channel quality indicator.
 24. The transmitter of claim 1 further adapted to: for at least some of the OFDM slots, partition OFDM sub-carriers transmitted during at least some of the OFDM slots between a plurality of receivers on a partitioning period basis.
 25. The transmitter of claim 24 wherein a partitioning period comprises one OFDM slot.
 26. A transmitter according to claim 25 further adapted to transmit a control channel identifying how each OFDM slot is partitioned between receivers such that each receiver can obtain its content.
 27. A transmitter according to claim 26 wherein the control channel comprises: identifiers for each receiver scheduled during the slot and an indication of which sub-carriers are for which receiver.
 28. The transmitter according to claim 27 wherein the control channel comprises: a first sub-channel and a second sub-channel; the first sub-channel containing an identifier of a first receiver and an identifier of a last OFDM sub-carrier for the first receiver; the second sub-channel containing an identifier of a second receiver, and an identifier of a last OFDM sub-carrier for the second receiver, the second sub-channel being only transmitted when there is a second receiver scheduled during a given slot.
 29. The transmitter according to claim 24 further adapted to perform sub-carrier and power allocation.
 30. The transmitter according to claim 29 adapted to perform sub-carrier and power allocation dynamically as a function of mobility and/or frequency selectivity.
 31. The transmitter according to claim 29 further adapted to perform sub-carrier and power allocation dynamically as a function of mobility and frequency selectivity by: defining conditions in which CDMA is used instead of OFDM as a function of channel quality and/or mobility and/or frequency selectivity; when conditions indicate OFDM is to be used: a) using a narrow frequency band and high power for high mobility, high frequency selectivity users; b) using a wide frequency band and low power for mid-mobility and mid-frequency selectivity users.
 32. The transmitter according to claim 29 further adapted to perform OFDM allocation subject to an overall power budget for an OFDM slot.
 33. The transmitter according to claim 1 adapted for use in a multiple carrier CDMA system.
 34. The transmitter according to claim 33 comprising: an OFDM modulator adapted to generate a single set OFDM sub-carriers that spans the multiple carriers, and in which a single wideband IFFT is performed to do OFDM modulation.
 35. The transmitter according to claim 33 wherein each carrier is allocated to only one of CDMA or OFDM on a per allocation period basis, and for any carrier that is allocated to CDMA during a given allocation period, zeros are inserted in a portion of the OFDM IFFT that overlaps the carrier during that allocation period.
 36. The transmitter according to claim 35 wherein the OFDM modulator is adapted to insert guardband sub-carriers between groups of sub-carriers for each carrier.
 37. The transmitter according to claim 1 wherein each OFDM slot comprises: a control portion which is non-OFDM followed by a plurality of IFFTs each separated by a respective prefix.
 38. The transmitter according to claim 37 wherein each IFFT is a 128 chip IFFT, and each prefix is 22 zero chips.
 39. The transmitter according to claim 1 further adapted to: adjust at least one power control parameter prior to transmitting OFDM slots so as to avoid/mitigate power control loop panic.
 40. The transmitter according to claim 39 wherein the at least one power control parameter comprises an outer loop power control threshold.
 41. The transmitter of claim 2 adapted to preferentially transmit CDMA slots in flat fading conditions.
 42. The transmitter of claim 2 adapted to preferentially transmit OFDM slots in dispersive conditions.
 43. A receiver comprising: an A/D converter adapted to produce a sequence of samples of received signal; CDMA demodulator for performing CDMA demodulation upon the sequence of samples; OFDM demodulator for performing OFDM demodulation upon the sequence of samples.
 44. The receiver according to claim 43 further adapted to receive a control channel identifying for a given slot whether or not the receiver is scheduled during the slot; wherein the receiver demodulates a given OFDM slot only if scheduled during the slot.
 45. The receiver according to claim 43, further adapted to: determine a CDMA channel quality indicator for CDMA and an OFDM channel quality indicator for OFDM; feed back a better of the CDMA channel quality indicator and the OFDM channel quality indicator; if the CDMA channel quality is better, use CDMA mode to demodulate a next slot for the receiver; if the OFDM channel quality is better, use OFDM mode to demodulate a next slot for the receiver.
 46. The receiver according to claim 43 adapted to: perform CDMA demodulation by descrambling at least one common channel, de-spread and perform soft-de-mapping and FEC decoding during which the OFDM content will be substantially converted to AWGN; demodulate the OFDM slots by generating an interference term due to a CDMA component of the received signal, subtract the interference term from the received signal, perform OFDM demodulation on a remaining signal.
 47. The receiver according to claim 46 adapted to generate the interference term by: de-scrambling, de-spreading, soft de-mapping and FEC decoding (or hard decision), re-encoding, re-spreading, re-scrambling, convolving with an estimated channel response.
 48. The receiver according to claim 43 adapted to use OFDM pilots to generate channel estimates for both OFDM and CDMA.
 49. The receiver according to claim 43 adapted to perform channel estimation by: performing an FFT on the sequence of samples; performing a first transformation on a set of samples collectively output by the FFT; performing a per sub-carrier transformation on an output of the first transformation; performing a second transformation on outputs of the per sub-carrier transformations collectively to produce an OFDM channel estimate; and performing an IFFT on the OFDM channel estimate to produce a time domain channel estimate for CDMA finger searching.
 50. The receiver according to claim 43 adapted to use CDMA pilots for channel estimation for both CDMA and OFDM.
 51. The receiver according to claim 43 adapted to perform channel estimation by: performing CDMA searching and correlation to produce a time-domain channel response; performing an FFT on the time domain channel response; performing a first transformation on a set of samples collectively output by the FFT; performing a per sub-carrier transformation on an output of the first transformation; performing a second transformation on outputs of the per sub-carrier transformations collectively to produce an OFDM channel estimate; and performing an IFFT on the OFDM channel estimate to produce a time domain channel estimate for CDMA finger searching.
 52. The receiver according to claim 43 adapted to receive a signal on multiple carriers, the receiver comprising an FFT function adapted to perform an FFT for sub-carriers spanning the multiple carriers.
 53. A method comprising: generating at least one code separated CDMA (code division multiple access) channel(s) that are scrambled with a common scrambling code; generating an OFDM packet channel, the OFDM packet channel being divided into OFDM slots during which OFDM signals are transmitted with capacity allocated on a per OFDM slot basis, the OFDM packet channel not being scrambled by the common scrambling code; combining the code separated CDMA channel(s) and the OFDM channel into a combined signal and transmitting the combined signal.
 54. The method of claim 53 wherein the at least one code separated CDMA channel(s) comprises at least one continuous CDMA channels and a CDMA packet channel that is divided into CDMA slots during which CMDA signals are transmitted with capacity allocated on a per CDMA slot basis.
 55. The method of claim 53 wherein the at least one code separated CDMA channel(s) comprises a CDMA packet channel that is divided into CDMA slots during which CMDA signals are transmitted with capacity allocated on a per CDMA slot basis.
 56. The method of claim 53 further comprising for at least some of the OFDM slots, partitioning OFDM sub-carriers transmitted during at least some of the OFDM slots between a plurality of receivers on a partitioning period basis.
 57. The method of claim 53 further comprising performing sub-carrier and power allocation dynamically as a function of mobility and/or frequency selectivity.
 58. The method of claim 53 adapted for use in a multiple carrier CDMA system, the method further comprising generating a a single set OFDM sub-carriers that spans the multiple carriers, and in which a single wideband IFFT is performed to do OFDM modulation. 